It has become increasingly evident that cancer pathogenesis can involve a superfamily of small non-coding RNAs named microRNAs. While the oncogenic or tumor-suppressing functions of a number of microRNAs have been characterized, the role played by microRNAs in mediating metastasis was addressed only recently by work from our laboratory and several other groups. I am currently a Life Sciences Research Foundation Postdoctoral Fellow with the immediate goal of completing additional 1-2 years of mentored research with Dr. Bob Weinberg at the Whitehead Institute for Biomedical Research. In my initial screening, I identified three microRNAs that are most significantly upregulated in human breast cancer cell lines: miR-155, miR-9, and miR-l0b. Subsequently, I identified miR-l0b as a Twist-regulated microRNA that is highly expressed in metastatic cancer cells as well as in metastatic breast tumors from patients. This microRNA inhibits synthesis of the HOXD10 protein, permitting the expression of the pro-metastatic gene product, RhoC. Importantly, overexpression of miR-l0b in otherwise-non- metastatic breast cancer cells enables them to invade and metastasize in vivo. These findings represent the first functional evidence that overexpression of a microRNA can contribute to the development of metastasis. Since it remains unexplored whether targeting miR-l0b will inhibitmalignant progression, I now propose to combine genetic and pharmacological approaches to study miR-l0b loss-of-function effects in development and in breast cancer metastasis. Moreover, based on a previously undescribed observation that miR-l0b overexpression in the primary mammary tumor leads to lung macrophage accumulation prior to evidence of histologically detectable pulmonary micrometastases, I will explore the cell non-autonomous effects of miR-l0b in metastatic progression. In parallel, I will characterize the role of a second candidate microRNA that stood out in my initial screen, miR-9, in epithelial-mesenchymal transitions, angiogenesis, and metastasis, given that miR-9 is the only microRNA that is predicted to target E-cadherin, a major adhesion molecule whose loss is strongly implicatedin tumor invasion and metastasis. The long-term goal is to understand the role of microRNAs in regulating metastasis, and to develop new candidatetherapies for malignant diseases. RELEVANCE: Not only does this study illuminate the genetic and molecular basis of tumor metastasis, but it also has implications for breast cancer treatment. This will be the first proof-of-concept study of whether inhibiting a specific microRNA can prevent metastasis. Investigation of the cell non-autonomous effects of metastasis- mediating microRNAs may lead to discovery of endocrine proteins that have therapeutic potentials.